The present invention relates to an electron holography microscope which displays the three-dimensional structure, magnetic domain structure, etc. of a specimen in accordance with an electron holography method.
Recently, holography has been used in the field of electron microscopy similarly to the use of light, owing to the fact that a high-brightness electron gun in the form of a field-emission electron gun has been put into practical use. Electron beam holography is such that, as shown in FIG. 1, an electron beam 2' emanating from a high-brightness electron gun 2 in an electron microscope housing 1 is divided by an electron beam prism 4 into a specimen electron beam transmitted through a specimen 3 which contains information of the specimen, and a reference electron beam passing outside the specimen which functions as a reference of the electron beam phase. The two electron beams interfere on the focusing plane of the electron microscope so as to form an electron beam interference pattern 5. As a result, a fringe pattern, as illustrated in FIG. 2, appears on the focusing plane of the electron microscope, in superposition on a fine specimen image 6 which has been produced in prior holography microscopes. Whereas the conventional electron microscope specimen image 6 is a two-dimensional shadow image of the specimen, the fringe pattern gives three-dimensional information which is a function of electron beam shift in the specimen. More specifically, the interference pattern 7 of the part outside the specimen is a fringe pattern of straight lines having a period of M .lambda./sin .alpha. wherein M is the magnification of the electron microscope, .lambda. is the wavelength of the electron beam and .alpha. is the tilting angle of the electron beam as shown in FIG. 2. In contrast, the interference pattern 6 of the part transmitted through the specimen 3 deviates from the straight lines. The deviation is attributed to the fact that the electron beam has its phase disordered when transmitted through the specimen. The quantity .rho. of the deviation is proportional to the thickness of the specimen in the corresponding part. Accordingly, when the parts in the interference pattern in which the quantities of deviation are a fixed value are traced, a contour line map concerning the thickness of the specimen is obtained, and a three-dimensional structure is produced.
In prior-art electron holography microscopes, very complicated operations were performed to obtain the contour line map for the reason that the interference pattern is formed by the electron beam in the vacuum microscope housing. More specifically, the electron beam interference pattern is recorded on a photographic film in the vacuum camera chamber of the electron microscope, and the photographic film is taken out of the electron microscope. The photographic film is developed, fixed and dried in a darkroom, to obtain a hologram film which can be optically processed. Next, the film is set on a holographic picture reconstructing apparatus which employs a laser source, an optical lens-system and an optical prism. The conditions of interference of the laser beam are adjusted to obtain a reconstructed hologram image which expresses the contour line map. Thus, the aforementioned series of operations in the prior art electron holography microscope have required a period of time of nearly one day or more and involved complicated operations such as the darkroom processes and adjustments of the holographic picture reconstructing apparatus. The time necessary to obtain a holographic picture in the prior art system has been especially disadvantageous where the immediate investigation of a three-dimensional structural variation of the specimen is required.